1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM:Wavelength Division Multiplexing) optical communication technique. In particular, the present invention relates to a WDM optical communication system and a WDM optical communication method, wherein pre-emphasis is performed on optical signals of respective wavelengths depending on reception information measured at the receiving end.
2. Description of Related Art
In recent years, research and development of WDM optical communication systems have been carried on actively with an aim toward a large transmission capacity of communication lines. With conventional WDM optical communication systems, it is known that deviation of transmission characteristics occurs in optical signals of respective wavelengths. The transmission characteristics of a WDM optical communication system can be measured, for example, by the SN ratio of the optical signal (OSNR). However, due to the abovementioned deviation of transmission characteristics, the OSNR of an optical signal of a specific wavelength is worse in a wavelength division multiplexed optical signal, so that, in spite of there being an optical signal of a wavelength with a good OSNR, the transmission characteristics are estimated by the worst OSNR. Consequently, in a WDM optical communication system, it is required that the OSNR of optical signals of any wavelength should show similarly good value.
The main causes of deviation of transmission characteristics as mentioned above are known. For example, due to wavelength dependences such as the gain of optical amplifiers installed in repeaters and losses in optical fiber transmission paths, or due to transmission and reception level deviations between respective wavelength caused by the Raman effect of an optical fiber and the like, the OSNR of an optical signal of a specific wavelength is degraded at the receiving end.
To counteract the abovementioned OSNR degradation, the application of a pre-emphasis technique is effective, for example. With a pre-emphasis technique, photoelectric power of each channel (an optical signal of each wavelength) on the transmitting end is set to a different value. To be specific, pre-emphasis is performed on a channel whose OSNR at the receiving end is low in the case where pre-emphasis is not performed, in order to set higher photoelectric power at the transmitting end than that of the other channels. Furthermore, pre-emphasis is performed on a channel whose OSNR at the receiving end is high in the case where pre-emphasis is not performed, in order to set lower photoelectric power at the transmitting end than that of the other channels. In this manner, the deviation of the OSNR between channels can be suppressed.
Furthermore, another main cause of deviation of transmission characteristics is known in that, for example, in a WDM optical communication system including optical add drop nodes, variation of the OSNR occurs at the receiving end due to a difference in the number of optical amplifiers that each channel passes through. That is to say, a channel with many transmission spans has a degraded OSNR at the receiving end, while a channel with a small number has a margin in the OSNR at the receiving end.
For the variation of the abovementioned OSNR, a method has been developed, for example, wherein at each optical add drop node, the input light level of each wavelength, noise figure data of the optical amplifier, and the like are collected, and an optimal pre-emphasis amount is set for each optical path through which an optical signal of each wavelength passes.
However, in a conventional WDM optical communication system wherein pre-emphasis is performed as mentioned above, there is a problem in that, if the pre-emphasis amount to be applied to the WDM signal light increases, the influence of nonlinear optical effects increases, so that the transmission characteristics are degraded. Such degradation due to the influence of nonlinear optical effects is difficult to determine accurately by monitoring only the OSNR conditions at the receiving end. Therefore, there is a case where optimal transmission characteristics cannot be obtained, even though pre-emphasis is controlled depending on the OSNR monitored at the receiving end in a conventional manner.
Furthermore, there is another problem in conventional WDM optical communication systems in that, if the range of waveband of WDM signal light is expanded, deviation of the transmission characteristics is difficult to limit sufficiently by only the pre-emphasis control performed at the transmitting end. For example, techniques aimed at expanding the optical signal waveband range by applying such as a Raman amplifier to an optical repeater station and the like have been developed. However, there has been a case in that, if pre-emphasis of an optical signal with such a broad waveband is to be controlled only at the transmitting end, a required amount of pre-emphasis control becomes difficult to be performed owing to the control range constraints of variable optical attenuators and the like that control the power of transmitted light. To deal with such situations, it is considered to be effective to use a combination of other control techniques that control the deviation of transmission characteristics with pre-emphasis.